The invention is directed to a measuring circuit which can measure impedance values. An important application of such measuring circuits consists in determining, together with inductive displacement transmitters, regulating distances, whether in rotational or in linear movement, of any structural component parts. Conventional inductive displacement transmitters are half-differential short circuit ring displacement transmitters, differential choke displacement transmitters and differential transformer displacement transmitters.
In order to measure the impedance values of structural component parts not exclusively showing ohmic resistance, it is necessary to apply alternating voltage to the structural component parts. The amplitude and often also the phase of the voltage at the structural component part are then measured. The measurement signal is generally smoothed by a low-pass filter to obtain a direct voltage as final measurement value. Measuring circuits, such as are preferably used with inductive displacement transmitters, are generally constructed in such a way that the emitted direct voltage is proportional to the regulating distance of a final control element of the displacement transmitter. It is very difficult to achieve highly accurate results with conventional measuring circuits, since the low-pass filters which are employed lead to phase shifting which is difficult to take into account and/or since the obtained direct voltage still has a residual ripple.
Accordingly, it has long been a problem to further develop measuring circuits, which are particularly suited for evaluating the signals of inductive displacement transmitters, in such a way that they emit a measurement signal of particularly high accuracy which is a measure for the impedance value of a variable impedance.